Thermionic valve circuits



June 19, 1945. P. K. CHATTERJEA ETAI, '.'2,378,620-

THERMIONIC VALVE CIRCUIT Filed May 20, 1943 Patented June 19, 1945 'rnEnMIomo VALVE-CIRCUITS- Prafulla Kumar Chatterjea and Charles Thomas Scully, London, England, assignors to Standard Telephones andaCables Limited, Iiondon, England; a British company Application May 20, 1943, Serial No. 487,695 In Great Britain July 1942 3: Claims. (G1. 250'--20) The present invention relates to automatic volume control arrangements for radio receivers and in particular makes use. of thermally sen.-

sitive re sistanceelement's known as thermistors foramplifyingtheicontrol'voltage. v

j Thermistors have been in use for' some years and are characterised by a temperature co-v efficient" of resistance which. may be either positive or. negative and which is moreover many I times the correspondingcoeflicient for a' pure metal such as copper. This property renders thermistors' particularly'suitable for a variety of special applications in electric circuits; 's

- Various different materials are available for the resistance; element' of a thermistor; these vari'ous" materials having" different" properties in other'respects; as one example; a resistance material having a high negativetemperature coefiicient of resistancecomprises a mixture of manganese oxide and nickel oxide, with or without the addition of certain other metallic oxides,

the mixture being suitably heat treated.

'L'h'ermistors have been employedintwo different forms: (a) known as a directly heated thermistor and comprisinga'resi'stanc'e element of the thermally sensitive resistance material provided with suitable lead-out conductors 0r terminals, andfb) known asan indirectly heated thermistor comprising theelement 6w) provided in addition witha heating" c'oil' electrically insulated' from the element. A- directly heated thermistor is-primariI'y intended to be" controlled by the-current which flows through it andwhic'h' varies the temperature and also theresistanceaccordingly. 'Such a thermistor'will also beaf fected by the temperature of its surroundings and may therefore be used forthermostati'c con 'trol and like purposes with or without direct thermistors Will be found in'zan article by G L.

Pearson. in the Bellv Laboratories Record; Dec;

1940, page106.

In radio receivers provided with automatic volume control it isgenerally desirable to providemeans for amplifying'thecontrol voltage desecondary Winding of which is tuned by an. ad-

tain a better adjustment of. the output. level; It is,v however, inconvenient. and. expensive. to provide additional' valves for this purpose, but it has been found that the necessary amplificationcan be very easily and. cheaply obtained by the us of indirectly heated thermistors.

According to the invention, therefore, there is provided an automatic volume control arrangement for a radio receiver comprising a first indirectly heated thermistor having its resistance element connected. in series with a first constant resistance. to a direct'current source, the heating.- coil. of the thermistor being connected in seriesbetween the positive terminal of the source and the plate of a first amplifying" valve, the

plate Cllll81'1t' .0f which. is adapted to be varied in accordance with the level of the incoming signals,. the volume control voltage being obtained from the junction point of the said resistance element and the said constant resistance;

' The invention will be more clearly understood from the following detailed description of an embodiment with reference to the accompanying drawing which showsa schematic circuit diagram thereof. This diagram shows part of the circuit of a typical radio receiver to which the invention is applied; It showsarectifying diode D from which the audio' frequency signals are obtained, at first amplifying valve Vr which iir also used for obtaining the volume controlvolt age; and: affinal amplifying valve V2 which supplies the signals'toaloud-speaker S through an output transformer OT. The signals arrive at terminals l' and 2 from other'parts of the'radio receiver circuit, which are not shown, and are applied to the left-hand plate circuit ofthe diode. D through an input, transformer IT,.the

justable. condenser (31.. The rectified signals are obtained. acrosszthe load; resistance R2; a smoothing circuit R1, C2, C3 being provided to remove the high frequency from the resistance R2. 'The rectified audio frequency signals. are applied to the control grid of valve V1 throughv the blocking condenser Cs' and resistance chain comprising: a potentiometer P and two resistances R3 and R4. By" adjusting the potentiometer P the proportion of the rectified signal voltageapplied'" to th gridmay be varied.

The incoming signals ar alsoapplied; to the right-hand plate of the diode through the condenser Cr and produce a voltage drop across the-resistanceRi in a negative sense which depends on the level of the signals; so that the rived from the-incoming signals inorder to on control grid of the valve Viis givena negative bias which increases as the signal level increases. The valve V1 is coupled to the valve V2 through a blocking condenser C7 and potentiometer comprising the resistance element R11 of an indirectly heated thermistor T1and a constant resistance Re, the junction point of R11 and Rs being connected to the control grid of V2. The heating coil 11 of the thermistor T1 is connected in series with the plate circuit of the valve V1, which circuit also includes a resistance Raa bypass condenser Ca being provided. The cathode of the valve V2 is biassed by means of a resistance R7 with the usual bypass condenser C9.

A high tension supply for the valves has its positive terminal connected to terminal 4 and its negative terminal to terminal 3. The plate circuit of valve V1 is supplied from terminal 4 through the heating coil ?"2 of a second thermistor T2 and also through T1 and R5 as previously explained. The plate of the valve V2 is supplied from terminal 4 in series with the primary winding of the output transformer OT. The negative terminal 3 is connected to ground through the field coil L of the loud-speaker S. This provides the necessary excitation, and the inductance of the coil, together with the condensers C11 and C12 provides a smoothing filter. Owing to the resistance of the-coil L the terminal 3 will be at a negative potential with respect to ground which might be as much as 100 volts.

Connected across the terminals 3 and 4 is a potentiometer comprising resistance R3 and the resistance element R12 of the thermistor T2. The junction point of R3 and R12 is connected through a resistance R9 to a terminal 5 where the automatic volume control voltage is obtained, and is supplied to the appropriate point or points in that portion of the radio receiver circuit which is not shown. The condenser C10 acts as a bypasscondenser for smoothing this voltage.

It is, of course, not essential to provide the excitation current for the coil L in this manner, and it can if desired, be provided from a separate source of supply. It is, however, desirable that terminal 3 should have a negative potential with respect to ground and, accordingly, if the coil L is separately supplied a suitable resistance should be included instead between terminal 3 and ground.

When there are no signals arriving in the receiving circuit the grid bias voltage of the valve V1 obtained from the resistance R3 will be small or zero and the plate current of this valve will be a maximum. Accordingly both the thermistors T1 and T2 will be hot. These thermistors should have anegative temperature coefficient of resistance and their resistance will accordingly be low. When signals at a high level are received a large negative bias voltage will be applied to the valve V1 and its plate current will fall, so that both the thermistors will cool and their resistance will increase. It will be evident that the potential of the terminal 5 will depend upon the resistance of the thermistor T2 and, therefore, also upon the level of the incoming signals. It will be preferable to choose the value of the resistance Ra and the value of R12 when the thermistor is hot, so that the potential of the therminal 5 with respect to ground is just sufficiently negative to provide the normal bias for the valve or valves which are being controlled. When the signal level increases the terminal 5 will become more negative and will accordingly increase the negative bias supplied to these valves and will reduce the gain of the circuit accordingly.

The action of the thermistor T1 is different from that of T2 since it operates to vary the coupling between the valves V1 and V2 instead of to provide a control voltage. This thermistor should also have a negative temperature coefficient of resistance. When there are no incoming signals it will be hot and its resistance R11 Will be small, and accordingly the proportion of the signal voltage applied to the control grid of valve V2 Will be large. When signals at a high level are received the thermistor T1 will cool and its resistance will increase and the proportion of the signal voltage applied to the valve V2 will be reduced.

It will be evident that the thermistors T1 and T2 are quite independent in their operation and T1 can be omitted if desired, the element R11 being replaced by a suitable constant resistance.

By asimple rearrangement of the circuit, thermistors having a positive temperature coefiicient of resistance can be used if desired. To enable this to be done it is only necessary to interchange RB and R12, and Re and R11 and the efiect will be substantially the same. In the condition when no signals are being received both thermistors will be hot as before, their resistance will be high instead of low so that substantially the same voltage variations at terminal 5 or atthe control grid of V2 will be obtained when the signal level varies.

Although the valves V1 and V2 have been shown for simplicity as triodes, it is obvious that other types of valve could be used if desired and in many cases it will be preferable to use pentodes. In such cases the heating coils of the thermistors could, if desired be connected in series with the screen grid of the Valve V1, so that they would be heated by the screen grid current, which would also vary with the signal level in a manner similar to the plate current.

It will be understood that the circuit of the accompanying figure which has been described is only one particular case where the invention can be applied and various modifications of the details are clearly possible.

It will be seen that a valve V1 can be regarded as acting both as an amplifier fOr the audio frequency signals and also as an amplifier for the continuous voltage which is developed across the resistance R3 and which signifies the level of the incoming signals. It will be clear that if desired, a separate valve could be used for amplifying this ,continuous voltage, in which case the thermistor heating coils would be connected in series with the plate circuit of this separate valve.

What is claimed is:

1. An automatic volume control device for a radio receiver including a first amplifying valve and an indirectly heated thermistor having a heating coil and having a resistance element, a first constant resistance connected in series with said resistance element, a direct current source feeding both resistance in series, means connecting the heating coil of the thermistor in series between the positive terminal of said source and the plate of said first amplifying valve, the plate current of which valve is adapted to be varied in accordance with the level of the incoming signals. and means for obtaining the volume control voltage directly from the junction point of said resistance element and said constant resistance.

2. Device according to claim 1 further including a second amplifying valve and a second indirectly heated thermistor having a resistance element and a heating coil, a second constant resistance, said second thermistor resistance be ing connected in series with saidsecond constant resistance between said plate of said first ampli-- eluding a third resistance connected between the negative terminal of said direct current source and ground, the resistance element of said first thermistor, said first constant resistance, and the value of said third resistance being so chosen that the volume control voltage is slightly negative when no signals are being received and becomes increasingly negative as the level of incoming signals increases.

PRAFULLA KUMAR CHAT'I'ERJEA. CHARLES THOMAS SCULLY. 

